Method for crystal growth control

ABSTRACT

The growth of a crystalline body of a selected material is controlled so that the body has a selected cross-sectional shape. The apparatus is of the type which includes the structure normally employed in known capillary die devices as well as means for observing at least the portion of the surfaces of the growing crystalline body and the meniscus (of melt material from which the body is being pulled) including the solid/liquid/vapor junction in a direction substantially perpendicular to the meniscus surface formed at the junction when the growth of the crystalline body is under steady state conditions. The cross-sectional size of the growing crystalline body can be controlled by determining which points exhibit a sharp change in the amount of reflected radiation of a preselected wavelength and controlling the speed at which the body is being pulled or the temperature of the growth pool of melt so as to maintain those points exhibiting a sharp change at a preselected spatial position relative to a predetermined reference position. The improvement comprises reference object means positioned near the solid/liquid/vapor junction and capable of being observed by the means for observing so as to define said reference position so that the problems associated with convection current jitter are overcome.

The Government has rights in this invention pursuant to Contract No.NAS7-100, JPL Subcontract 954355 awarded by the U.S. Department ofEnergy.

This application is a division of our copending application Ser. No.915,443, filed June 14, 1978 for Method and Apparatus for Crystal GrowthControl.

This invention pertains to the growth of crystalline bodies having apredetermined cross-section and more particularly to improvements inapparatus for and processes of growing such crystalline bodies.

Various processes are now known for growing crystalline bodies. One suchprocess, hereinafter referred to as the "capillary die process,"generally utilizes a capillary die or forming member from which thecrystalline body can be grown. The process can be carried out inaccordance with various techniques. By way of example, one suchtechnique is described in U.S. Pat. No. 3,591,348 in which bodies aregrown in accordance with the edge-defined film-fed growth technique(also known as the EFG Process). In this technique the cross-sectionalshape of the crystalline body is determined, in part by the external oredge configuration of the top end surface of the capillary die member.The process involves the growth of a seed from a liquid film of feedmaterial sandwiched between the growing body and the top end surface ofthe die member with the liquid in the film being continuouslyreplenished from a suitable melt reservoir via one or more capillariesin the die member. By appropriately controlling the pulling speed of thegrowing body and the temperature of the liquid film, the flim can bemade to spread (under the influence of the surface tension at itsperiphery) across the full expanse of the top end surface formed by theintersection of that surface with the side surface or surfaces of thedie member. The growing body grows to the shape of the film whichconforms to the edge configuration of the die member's top end surface.Since the liquid film has no way of discriminating between an outsideedge and an inside edge of the die's top end surface, a continuous holemay be grown in the crystalline body by providing in that surface ablind hole the same shape as the hole desired in the body, provided,however that any such hole in the die member's top end surface is madelarge enough so that surface tension will not cause the film around thehole to fill in over the hole.

Another example of the capillary die process for growing crystallinebodies is described in U.S. Pat. No. 3,471,266. This technique employs aforming or die member that defines a capillary which contains a columnof melt from which a crystalline body is grown and pulled. Dependingupon the cross-sectional configuration of the capillary and byappropriate control of the thermal conditions in the upper end of thecolumn of melt contained in the capillary, it is possible to growcrystalline bodies of selected material having arbitrary selectedcross-sectional shapes. Thus, by employing a forming die member having acapillary in the shape of an annulus, it is possible to grow a hollowtube. The forming member is mounted so that the capillary is connectedto a reservoir pool of melt, whereby the capillary is self-filling.

In capillary die processes, such as the two types described, changes inpulling speed and the temperature distribution, i.e., thermal gradientsat the top end of the die member and in the melt near the solid/liquidgrowth interface (formed where the growing crystalline body joins theliquid film) can affect the cross-sectional size of the growing body.Since it is a relatively easy matter to hold the pulling speed constant,the usual practice, once the crystalline body is growing to the desiredshape, is to fix the pulling speed at a suitable rate and to adjust theinterface temperature gradients (by adjusting the rate of heating) sothat the body will grow to the desired size.

Accordingly, it is desirable to monitor the growing body so as to keepthe temperature gradients and the pulling speed within prescribedtolerance limits. Several monitoring systems and techniques are known.For example, a technique described in U.S. Pat. No. 3,870,477 issued toLaBelle is predicated upon the fact that capillary die processes ofgrowing crystalline bodies are characterized by the presence of ameniscus of melt extending between an edge of the die member and thesolid/liquid growth interface. The height of (and the degree ofconcavity) of the meniscus can change with changes in the operatingconditions. More importantly, the height of the meniscus is affected bythe temperature of the melt in the region of the solid/liquid growthinterface and the pulling speed, and that the outer diameter of a hollowtube or solid rod or the thickness and width of a flat ribbon willdecrease as the meniscus height increases (and an increase in the samediameter or width occurs if the outer meniscus height decreases).

A system described in U.S. copending application Ser. Nos. 778,577 and778,589 (both filed Mar. 17, 1977 and assigned with the presentapplication to a common assignee), now respectively U.S. Pat. Nos.4,185,076 and 4,184,907, both issued on Jan. 22, 1980, monitors meniscusheight by taking advantage of the discovery that during capillary dieprocesses the meniscus joins the crystalline body at thesolid/liquid/vapor junction (the junction formed at the intersection ofthe solid-liquid interface and the surrounding vapor) at a discernablemeniscus angle. The term "meniscus angle," designated hereinafter as φ,shall therefore mean the angle formed by the meniscus surface (theliquid/vapor interface) with the surface of the solid crystalline body(the solid/vapor interface) at the solid/liquid/ vapor junction. Duringthe growth of a uniformly dimensioned crystalline body, i.e. duringsteady state conditions when the pulling speed of the crystalline bodyas well as temperature gradients near the solid/liquid growth interfaceare substantially constant, the surface of the crystalline body, i.e.the solid/vapor interface is parallel to the pulling axis. It has beenfound that for at least some materials, such as silicon and germanium,during these steady state conditions, the "steady-state meniscus angle"φ_(o) is substantially constant at some unique value depending upon thematerial (see Surek, T. and Chalmers, B; "The Direction of Growth of theSurface of a Crystal in Contact with Its Melt;" Journal of CrystalGrowth; Volume 29, pp. 1-11 (1975)).

For silicon, for example, the steady state meniscus angle is 11°±1°,while for germanium approximately 8°. It has further been observed that,at least for silicon, the steady state meniscus angle will not beappreciably affected by variations of nearly two orders of magnitude inthe crystal growth rate even though changes in meniscus height willoccur.

The system described in U.S. copending application Ser. Nos. 778,577 and778,589 now respectively U.S. Pat. Nos. 4,185,076 and 4,184,907; whichutilizes meniscus height control accordingly comprises means forobserving during a capillary die process at least a portion of the solidcrystalline body and the meniscus including the solid/liquid/vaporjunction, for quantities of radiation (at wavelengths to which thecrystalline material is at least partially reflective) which areinherently provided within the growing apparatus due to the hottemperature atmosphere and reflected in a direction substantiallyperpendicular to the meniscus surface formed at the solid/liquid/vaporjunction when steady state conditions exist. By determining the spatialposition where a sharp change (or contrast) in the reflected radiationoccurs, and referencing that position relative to some arbitraryreference spatial position (determined electronically by adjusting aninput voltage potential level), the speed at which the crystalline bodyis pulled or the temperature of the growth pool of melt can bemaintained so that the position exhibiting the sharp contrast can becontrolled relative to the reference position. Although maximum contrastis achieved by observing the solid/liquid/vapor junction at an angleperpendicular to the surface of the meniscus at the junction, themeniscus height control technique can be accomplished by observing thesurface at the junction at an angle which varies from the perpendicularso long as a discernable contrast arising from the reflected radiationfrom the meniscus and body surfaces exists.

Once steady state conditions are achieved, it is desirable to maintainthe position of the solid/liquid/vapor junction relative to, forexample, the top of the die member to within ±1 mil so as to maintainsteady state growth conditions. Although the meniscus height controlsystems are sensitive enough to provide such control, it has been foundthat, at least in some furnaces for growing crystalline bodies, heatconvection currents between the actual object being viewed and thesensing instrument can produce "convection current jitter". The latterproduces shifts in the image received by the instrument due to changesin the index of refraction produced by the convection currents when, infact, no shift in the solid/liquid/vapor junction has occured. Thus,false adjustments will result in the paramater or parameters beingcontrolled by the instrument.

Accordingly, it is an object of the present invention to provide animproved apparatus for and method of monitoring and controlling crystalgrowth so that the cross-sectional dimensions of the growing body iswithin prescribed limits.

Another object of the present invention is to provide an improvedapparatus for and method of automatically monitoring and controlling thegrowth of crystalline bodies, such as silicon, so that thecross-sectional dimensions of the growing body are substantiallyconstant.

A further object of the present invention is to provide an improvedapparatus for and method of controlling and monitoring growth ofcrystalline bodies which includes all of the advantages of the meniscusheight control system and technique disclosed in U.S. copendingapplication Ser. Nos. 778,577 and 778,589, now respectively U.S. Pat.Nos. 4,185,076 and 4,184,907 and described hereinafter.

And yet another object of the present invention is to provide animproved apparatus for and method of controlling and monitoring thegrowth of crystalline bodies in crystal-growing furnaces while avoidingthe above-noted problems associated with convection current jitter.

The foregoing and other objects of the present invention are achieved byimproved crystal growing apparatus of the type described in copendingapplication Ser. Nos. 778,577 and 778,589, now respectively U.S. Pat.Nos. 4,185,076 and 4,184,907 which employs a capillary die process andcomprises means for observing at least a portion of the solidcrystalline body and the meniscus including the solid/liquid/vaporjunction, for quantities of radiation (at wavelengths to which thecrystalline material is at least partially reflective) reflected in adirection substantially perpendicular to the meniscus surface formed atthe solid/liquid/vapor junction when steady state conditions exist. Thespatial position where a sharp change (or contrast) in the reflectedradiation occurs is determined and this spatial position is referencedrelative to some reference spatial position so that the speed at whichthe crystalline body is pulled or the temperature of the growth pool ofmelt is maintained is controlled so that the position exhibiting thesharp contrast can be controlled relative to the reference position. Theimprovement comprises reference object means fixed in space, near thesolid/liquid/vapor junction for defining the reference position so thatsaid means for observing also observes the reference object means and sothat shifts in the image of said solid/liquid/vapor junction as seen bysaid means for observing, caused by conviction current jitter, willproduce corresponding shifts in the image of said reference means.

Other features and specific details of the present invention are setforth in the following description which is to be considered togetherwith the drawings wherein:

FIG. 1 is a sectional view in elevation, of a crystal growing furnace,incorporating the present invention, with certain parts representedschematically, and illustrates the growth of a crystalline bodyaccording to a capillary die process;

FIG. 2 is a schematic view on an enlarged scale illustrating a portionof the apparatus shown and described in copending application Ser. Nos.778,577 and 778,589, now respectively U.S. Pat. Nos. 4,185,076 and4,184,907 as combined with the furnace of FIG. 1;

FIG. 3 is a schematic block diagram illustrating further aspects of theembodiment described in copending application 778,577 and 778,589, nowrespectively U.S. Pat. Nos. 4,185,076 and 4,184,907;

FIG. 4 is an exemplary timing diagram of the block diagram of FIG. 3;

FIG. 5 is a schematic view illustrating a portion of the apparatusdescribed in copending application Ser. Nos. 778,577 and 778,589, nowrespectively U.S. Pat. Nos. 4,185,076 and 4,184,907 as modified inaccordance with the present invention;

FIG. 6 is a schematic block diagram illustrating the modifications tothe embodiment of FIG. 3 to incorporate the principles of the presentinvention; and

FIG. 7 is an exemplary timing diagram of the block diagram of FIG. 6.

Like numerals are used to indicate like parts in the several figures.

Capillary die processes, of the character already described, arecharacterized by the presence of a vertical meniscus of melt extendingbetween en edge of the die or forming member and the liquid/solid growthinterface. It is generally known that the vertical height of themeniscus can change with changes in the operating conditions. When achange in meniscus height occurs the solid/liquid/vapor junction formedby the intersection of the liquid meniscus and solid crystalline bodyshifts its position relative to some fixed reference position such asthe top of the die member. Since the vertical height of the meniscus isaffected by the temperature of the melt in the region of the growthinterface as well as the pulling speed, it necessarily follows that ashift in the vertical position of the solid/liquid/vapor junctionrelative to this fixed reference position indicates a change inoperating conditions. Thus, the cross-sectional dimensions of thecrystalline body will increase or decrease depending on whether thesolid/liquid/vapor junction shifts toward or away from the fixedreferenced position. As described in copending application Ser. Nos.778,577 and 778,589, now respectively U.S. Pat. Nos. 4,185,076 and4,184,907 where the crystalline material is at least partiallyreflective to preselected wavelengths of radiation, the cross-sectionaldimensions of the crystalline body can be monitored and controlled bytaking advantage of the fact that a unique steady state meniscus angleexists at the solid/liquid/vapor junction during steady stateconditions.

More specifically, it may be observed that when viewing a flat surfacealong an axis substantially perpendicular to the surface through forexample, a long tube, substantially all radiation reflected off thesurface at an acute angle will not be seen and consequently the surfacewill appear dark (due to the apparent absence of the radiation).However, as the viewing angle changes with respect to the perpendicular,the dark spot seen through the long tube will lighten as a result ofreflected radiation being directed along the viewing axis. It has beenobserved that this phenomenon will occur even when viewing a hot objectsince in addition to radiation emitted by the hot object, it may alsoreflect radiation emitted by other radiation emitting and reflectingobjects which are in the immediate vicinity of the hot object.

As described in copending application Ser. Nos. 778,577 and 778,589, nowrespectively U.S. Pat. Nos. 4,185,076 and 4,184,907 by viewing at leasta portion of the crystalline body and meniscus surfaces, including thesolid/liquid/vapor junction, in a direction substantially perpendicularto the meniscus surface formed at the solid/liquid/vapor junction whensteady state conditions exist, a sharp contrast or change in radiationintensity can be recognized at the solid/liquid/vapor junction. Thiscontrast occurs since the viewing axis is perpendicular to the meniscussurface at the solid/liquid/vapor junction and thus substantially allradiation reflected off the meniscus at the junction will be reflectedat an acute angle to the viewing axis and thus will not be seen.Conversely, the surface of the solid body will be at an angle, i.e. themeniscus angle, to the surface of the meniscus so that the viewing axiswith respect to the surface of the body at the junction is less than90°. For silicon, for example, during steady state growth this anglewill be 79°±1°. Some reflected radiation, therefore, can be seen fromthe crystalline body providing a contrast between the two surfaces atthe solid/liquid/vapor junction.

The above is best illustrated in connection with the drawings whereinFIG. 1 shows a crystal growing furnace 10 provided with a suitablysupported crucible 12. The crucible contains a crystalline melt material14 from which the crystalline body 16 is to be grown, the melt materialbeing maintained at a predetermined temperature by one or more heaterelements 18. A suitable capillary die or forming member 20 is supportedby the plate 22 resting on the crucible so that one end 24 of the diemember extends into the melt material 14 while the other end 26 of thedie member extends above the plate 22. The capillary die member 20, asshown, is similar to the type of the die member employed in the EFGprocess previously mentioned and described in detail in U.S. Pat. No.3,591,348. Generally, the cross-sectional shape of the crystalline body16 is determined by the external or edge configuration of the upper end26 of the die member 20. By way of example, the die may be designed forgrowing a thin flat ribbon, in which case FIG. 1 may be considered aspresenting a side edge view of the die with the longer horizontaldimension of the ribbon, i.e., its width being perpendicular to theplane of the drawing. As shown more clearly in FIG. 2, the die member 20includes at least one capillary 28 so that the liquid in the meniscusfilm 29 formed between the top of the die member 20 and the crystallinebody 16 can be continuously replenished from the reservoir of meltmaterial 14 as the body 16 is being pulled. The body 16 is pulled at aconstant speed along a pulling axis by the pulling mechanism 32. Inorder to provide a more uniform temperature of the meniscus duringsteady state conditions, a plurality of thin radiation shields 34 areprovided on the plate 22 around the die member 20 so as to help maintaina uniform temperature of the melt material 14.

As shown more clearly in FIG. 2, the meniscus 30 of film 29 intersectsthe body 16 at the solid/liquid interface 36, which in turn forms thesolid/liquid/vapor junction 38. As previously described, during steadystate conditions (when the body 16 is being pulled at a constant speedand the temperature of melt material is substantially constant) thesolid/vapor interface 42, i.e. the surface of body 16, is parallel tothe pulling axis 40. By extending the interface 42, the meniscus angle φis formed with respect to the surface of the meniscus 30 at the junction38 as shown.

In accordance with the disclosures of copending applications Ser. Nos.778,577 and 778,589 now respectively U.S. Pat. Nos. 4,185,076 and4,184,907; at least a portion of the body 16 and meniscus 30 includingthe solid/liquid/vapor junction 38 is viewed at an angle perpendicularto the surface of the meniscus formed at the junction during steadystate conditions. Accordingly, the furnace 10 of FIG. 1 is provided withone or more viewing ports 44 provided with corresponding windows 46 andlocated so that portions of the body and meniscus can be viewed at thisangle along the viewing axis 48. It will be appreciated that since thepulling axis 40 is vertical, the viewing axis is preferably oriented atthe complement of the steady state meniscus angle φ_(o) with respect tothe pulling axis. Thus, for furnaces where silicon is to be grown, eachport 44 is located so that its viewing axis 48 intersects the meniscusat an approximate 11° angle with the horizontal or conversely at anapproximate 79° angle with respect to the pulling axis. The line ofdemarcation thus formed by the sharp change in contrast at thesolid/liquid/vapor junction 38 can be viewed with suitable means 50provided at one or more of the ports 44 along the corresponding axis 48to determine if the junction shifts relative to some fixed referencedposition which, as described in copending applications 778,577 and778,589, is electronically determined. When shifts occur, the speed atwhich the body 16 is being pulled or the temperature of the growth poolof melt 14 can be adjusted so that the line of demarcation indicatingthe solid/liquid/vapor junction 38 will return to its correct position.

As shown in FIG. 1 and more particularly in FIG. 2, the means 50includes a lens 52 properly mounted in a cool tube 51 (the latterpreferably is cooled by having a cooling fluid flowing therethrough) andpositioned along viewing axis 48. It will be appreciated thatalternatively lens 52 can be located outside of the furnace along theoptical axis 48. Lens 52 forms an image of at least a portion of thebody 16 and meniscus 30 in the image plane 53. The lens may be designedto provide any desired magnification of the image of the portion of thebody 16 and meniscus 30 depending upon the resolution and accurancy ofcontrol desired. A lens having a magnification of 2 has been foundsatisfactory although a lens providing anywhere from 1 to 5 timesmagnification would be equally satisfactory. A monitoring and controlsystem 55 is positioned with respect to the image plane 53 so that theposition of the solid/liquid/vapor junction 38 can be observed and thegrowth of the crystalline body 16 can be controlled.

Referring to FIG. 2, system 55 preferably includes a plurality ofradiation detectors 54 arranged in a linear array oriented in adirection perpendicular to the viewing axis 48 and coplanar with theimage plane 53. Detectors 54 are each of a type which provides anelectrical output signal, the magnitude of which is proportional to theamount of radiation, within a predetermined waveband, which is receivedby the detector. The waveband to which detectors 54 are sensitiveincludes radiation to which the body 16 and meniscus 30 are at leastpartially reflective. It will be appreciated therefore that the amountof reflected radiation within the wavebeand of reflected radiation thatis necessary for the present invention to operate in dependent in partupon the sensitivity of detectors 54 and the reflectivity of thecrystalline material at a particular wavelength. For example, siliconexhibits a high reflectivity, i.e., approximately 38%, at 0.5μ. Byemploying detectors which are sensitive to radiation at 0.5μ, theposition of the junction 38 can be ascertained by observing the positionof the discontinuous change in the amount of radiation at 0.5μ receivedby the detectors. Such detectors are commercially available. Forexample, "linear-imaging devices" (LIDs) or "linear-imaging sensors"(LISs), such as, for example, the CCD-110 system from the FairchildCamera Instrument Corporation of Mountain View Calif., can be used.

Referring to FIG. 3, the circuit is shown as including the CCD-110system generally referred to as sensing unit 60. In addition todetectors 54, the sensing unit includes an analog shift register (notshown) for simultaneously storing the output values of the individualdetectors, all of the latter sensing the radiation received at the sameinstant of time. The unit may include its own clocking circuitry or inthe alternative clocks 62 may be provided as shown. Unit 60 providesfour output signals from four corresponding output terminals. The firstoutput signal is hereinafter designated the transfer pulse φ_(A). Thesecond output is hereinafter designated the clearing or reset pulseφ_(B) and is provided subsequently to φ_(A). Both pulses are shown inthe timing diagram of FIG. 4. After the second output pulse, the unitthen provides a clock or strobe signal φs (which includes a series ofpulses) and a video output signal, both shown in the timing diagram ofFIG. 4. The video signal is essentially a series of pulsed analogsignals representative of the outputs of the corresponding detectorsstored in the analog shift register. However, the amplitude of thesignal is negative in polarity (assuming no D.C. bias) and proportionalto the intensity of the radiation detected by the particular detector.Thus, assuming no D.C. bias, the amplitude of the signal is zero in theabsence of radiation and of a negative amplitude proportional to theintensity of radiation when such radiation is present. By way ofexample, as shown in FIG. 4, when the 21st clock pulse is provided theamplitude of the video signal corresponds to the amount of radiationdetected by the 21st detector. Similarly, the amplitude of the videosignal occuring during the time of the 22nd clock pulse correspnds tothe amount of radiation detected by the 22nd detector. The video outputof unit 60 is connected to the positive input of a comparator 64. Thenegative input of comparator 64 is connected to an adjustable DC voltagesource. The latter provides the threshold level of the comparator.Generally, the output of comparator 64 is at a low or negative logicstate so long as the voltage at the positive input is below thethreshold voltage at its negative input. The output of comparator 64,however, changes to a high or positive logic state when the voltage atits positive input exceeds the reference voltage (threshold level) atits negative input. One-shot 65 having an input for receiving the clocksignal θs, has its output connected to the strobe input of comparator64, so that the latter only compares the amplitude of each pulse signalof the video output with the reference voltage. The output of comparator64 is connected to the clearing or reset input of a J-K flip-flop 66,with the set input of the latter being adapted to receive the θ_(B)output pulse from unit 60. Generally, flip-flop 66 operates so that itsQ output 70 goes to a high logic state (positive voltage) when the θ_(B)pulse is received at its setting input. The Q output will remain highuntil the reset input receives a positive voltage pulse from the outputof comparator 64, whereupon the Q output of the flip-flop will go to alow logic state (zero or some negative voltage) and remain in that stateuntil the next θ_(B) pulse is received at the reset input. The Q output70 of flip-flop 66 and the clocking output of unit 60 are connected totwo inputs of AND gate 74. The latter will provide an output in a highlogic state so long as its two inputs are high; otherwise the output islow. The output gate 74 is connected to the input of a scaler 76. Thelatter has its clearing input 78 and its transfer input 80 connected toreceive, respectively, the clearing pulse θ_(B) and the transfer pulseθ_(A) from the unit 60. Scalers are generally well known in the art,and, for example, the scaler 76 may be a frequency counter providing abinary output. Generally, scaler 76 counts the number of pulses of theclock signal received from the output of gate 74 when the latter isenabled by the Q output of flip-flop 66. This number is jam transferredto latch 82 when the transfer pulse θ_(A) is received at the transferinput 80 of the scaler 76.

The latch 82 generally holds this signal provided from scaler 76 untilan update is received from the scaler. Accordingly, latch 82 may be aregister or similar device. The digital output of latch 82 is convertedto an analog signal by the digital-to analog-converter 84. The analogoutput of converter 84 is signal conditioned by amplifier 86 to theswitch 88. The contact arm of switch 88 is connected to the summingjunction (not shown) of the controller 90. The controller is preferablyof the type manufactured under the trademark ELECTROMAX III by Leeds andNorthrup of North Wales, Pennsylvania through other controllers can beused. The summing junction of the controller 90 preferably sums theinputs from the output of amplifier 86, thermocouple 92, (the latterbeing positioned in crucible 12 or preferably near the end 26 of the diemember 20 in order to measure the melt temperature to be controlled) andthe input from a set point control 94. The output of controller 90 isconnected to an SCR controller 96. The latter provides an output whichis proportional to its input so as to provide a voltage suitable forcontrolling the heater elements 18 (shown in FIG. 1).

In operation, the means 50 is properly positioned with respect to port44 so as to observe the meniscus 30 at the steady state meniscus anglealong axis 48. Where silicon is being grown, axis 48 is oriented at 11°with respect to the horizontal, while germanium requires the axis to beoriented at about 8° with respect to the horizontal. As the crystal isgrowing, the monitoring system is designed to continually monitor andcontrol the location of the solid/liquid interface 36 relative to somefixed position such as the top of the die member 20. Referring to thetiming diagram of FIG. 4, the first pulse provided is the transfer pulseθ_(A). As will be more apparent hereinafter, the transfer pulse jamtransfers the value in the scaler 76 from the previous scan into latch82. The next pulse θ_(B) clears the scaler 76 for the next scan andresets flip-flop 66 so that the Q output 70 of the flip-flop goes highand gate 74 is enabled. As previously described, signals representativeof the amount of radiation received by the detectors 54 aresimultaneously stored in the analog shift register of sensing unit 60and subsequently provided as the series of pulse analog video outputsignals. With the scaler 76 cleared, a clock pulse θs is provided foreach video pulse shifted out of the analog shift register and providedat the video output of the unit 60. Each of these clock pulses iscounted in the scaler so long as the AND gate 74 is enabled. Theamplitude of the video signal is continually compared to the thresholdlevel set at the negative input of comparator 64. Since some reflectedradiation is received by the detectors positioned to receive radiationreflected from the crystalline body 16, the amplitude of thecorresponding portion of the video signal will be below the thresholdlevel so that the output of comparator 64 will remain zero, the Q output70 will remain high, the gate 74 will remain enabled and the scaler 76will continue counting the clock pulses.

A substantially reduced amount of reflected radiation is providedimmediately below the junction 38 as a result of viewing along theviewing axis 48. The amplitude of the video signal of unit 60 thereforebegins to rise. Although theoretically the amplitude of this signalshould increase almost instantaneously, as a practical matter thisgenerally does not occur. Instead as shown in FIG. 4, the amplitudeincreases over the next several pulses of the strobe signal (indicatinga decrease in detected radiation) since (1) the lens 52 utilized caninclude optical defects such as chromatic aberrations, (2) reflectionscan occur within the optical system such as multiple internalreflections within the window 46, when the latter is placed within theoptical system and (3) there is some radiation scattering from othersources which the detectors will sense as the reflected radiation ofinterest. Thus, by way of example, as shown in FIG. 4, the change occursat t1 so that the amplitude of the video signal begins to change. Whenthe amplitude of the video signal exceeds the threshold level at t2 theamplitude of the signal at the positive input of the comparator 64 willexceed the threshold level so that the output of the comparator will gofrom a zero or negative voltage to some positive voltage. The positivegoing transition of the voltage provided at the reset input of flip-flop66 will cause the Q output 70 to go from a high to a low state disablingthe AND gate 74. Once AND gate 74 is disabled, the scaler 76 willdiscontinue counting the strobe pulses. In the example shown in thetiming diagram, this count will be 26 corresponding to the previoustwenty-six strobe pulses. The video signal will continue until theentire analog shift register of sensing unit 60 is empty. It is notedthat after the interface has been detected even if the amplitude of thevideo signal should fall below the threshold level of comparator 64,causing the output of the latter to go to a zero or negative value the Qoutput 70 of flip-flop 66 will remain low and the AND gate 74 willremain disabled.

At the end of the video signal (when the values representative of allthe detectors have been provided) the transfer pulse θ_(A) is providedto the transfer input 80 of scaler 76 so as to jam transfer the value inbinary form from scaler 76 to latch 82 (in the example of FIG. 4, thisvalue being 26). The latch 82 will hold this value until the next scan(of the next set of readings sequentially provided from the analogregister) is completed and an update is provided from the output ofscaler 76. The value in latch 82 is a digital signal and applied to theinput of the digital-to-analog converter 84. The analog output ofconverter 84 is conditioned and shaped by amplifier 86 and applied tothe summing junction 90. If the solid/liquid/vapor junction 38 is in itsproper position the output of the summing junction and thus the outputof the controller 96 is such that the signal applied to the heatingelements is substantially constant and the temperature of the meltmaterial remains at the desired temperature as measured by thethermocouple 92. In order to properly set the voltage level of theoutput of amplifier 86 so that this remains true, the switch 88 isinitially set to ground and the output of the amplifier 86 is manuallyoffset to zero for ground level (by a potentiometer associated withamplifier 86) during steady state growth conditions. Thus, the outputfrom the summing junction 90 will represent the sum of the signals fromthe thermocouple 92 and the set point 94. In order to vary the steadystate temperatures of the melt temperature, the operator need only varythe setting of the set point 94 to provide a greater or lesser voltageto the heater elements during steady state conditions. The systemcontinually repeats the scan of the values in the analog register andtherefore periodically provides an update. Where a change in the numberof pulses counted in scaler 76 occurs when AND gate 74 is disabled, thenew value is jam transferred to latch 82, the analog output of converter84 will provide a larger or smaller signal to the input of summingjunction 90 so as to raise or lower the heat applied to the meltmaterial.

Various modifications can be made to the apparatus just described. Forexample, as shown in FIG. 5, the lens 52 can be replaced by a coherentoptical fiber bundle 100, whereby each fiber of the bundle transmits theamount of radiation received at one end of its other end. The latter endof each optical fiber of the bundle is preferably positioned adjacent acorresponding one of the detectors 54. Further, it will be appreciatedthat although maximum contrast is achieved by observing thesolid/liquid/vapor junction at an angle perpendicular to the surface ofthe meniscus at the junction, the angle can vary from the perpendicularso long as a discernable contrast arising from the reflected radiationfrom the meniscus and body surfaces exists.

It has been found through experience that the above-described apparatusis not always suitable in adequately controlling the meniscus height,and more particularly crystal growth control. For example, in somefurnaces heat convection currents between the actual object being viewedand the sensing instrument can produce "convection current jitter". Thelatter produces shifts in the image received by the sensing instrumentdue to changes in the index of refraction produced by the convectioncurrent when, in fact, no shift in the solid/liquid/vapor junction 38has occured. Since the reference position, to which the position ofjunction 38 is compared, is electronically determined, shifts in theimage of the junction 38, caused by convection current jitter,introduces errors. In order to correct for convection current jitter,the apparatus shown and described above is modified in accordance withthe present invention. Specifically, referring generally to FIG. 1 andmore particularly to FIG. 5, means in the form of a reference bar orobject 102, is fixed in space (by any suitable means such as bracket103, which in turn may be secured to a wall of furnace 10) adjacent thesolid crystal body 16 near the solid/liquid/vapor junction 38, so that aportion of body 16 (as shown in FIG. 5) or the meniscus 30 is observedby means 50 between the object 102 and the junction 38. The object 102is made of a material such that the amount of reflected radiation fromthe object and measured will contrast with the radiation reflected fromthe surface which is viewed as adjacent the object. As shown in FIG. 5this adjacent surface is provided by the body 16. In this situation, thedifference in contrast between the image of the reference bar 102 andthe image of the body 42 at the edge 104 of the reference bar can beused to reference the position of the solid/liquid/vapor junction 38.Any shift in the image of junction 38 due to convection currents willproduce an equal shift in the image of the edge 104 of the reference bar102.

In order to control the relative position of the junction 38 and theedge 104 of the reference bar, the control circuit of FIG. 3 is modifiedas shown in FIG. 6, wherein the comparator 64 is replaced by thecomparator circuit generally indicated at 106. As shown, the video inputsignal from the sensing unit is applied to the positive inputs ofcomparator amplifiers 108 and 110 and the negative input of comparatoramplifier 112. The negative input of comparators 108 and 110 and thepositive input of comparator 112 are provided with threshold levelvoltages by variable setting potentiometers, generally indicated at 114.Comparators 108, 110 and 112 are of the type which provide an invertedoutput. For reasons which will be more apparent hereinafter the settingsof the threshold level of the amplifiers are such that amplifier 112will normally provide a positive output so long as the amplitude of thevideo signal is above a first predetermined voltage, the output ofamplifier 108 will be zero or a negative voltage so long as the videosignal is above a second predetermined value (greater than the firstpredetermined value) and the output of amplifier 110 will be zero or atsome negative value so long as the video signal is above a thirdpredetermined value (less than the first predetermined value).

The outpts of comparators 108, 110 and 112 are connected to therespective inputs of AND gates 116, 118 and 120, respectively, whichoperate as video sampling gates. Specifically, the strobe signal θs isprovided at the input of one-shot 122. The output of the one-shot isapplied to a second input of each of the gates 116, 118 and 120. Eachpulse θs provided to one-shot 122 will produce in turn a pulse to theinput of gates 116, 118 and 120. Each gate provides a high output pulsewhen the input to the gate from the corresponding comparator and theone-shot are both high. The output of gates 116, 118 and 120 areconnected, respectively, to the inputs of AND gates 124, 126 and 128,which in turn provide outputs to the respective set inputs of flip-flops66, 130 and 132. A second input to gate 124 is connected to receive theQ output of flip-flop 130. The second input to gate 126 is connected toreceive the Q output of flip-flop 66, while the second input to gate 128is connected to the Q output of flip-flop 130.

The reset inputs of flip-flops 130 and 132 are both adapted to receivethe clearing pulse θ_(B) from sensing unit 60. The Q output of flip-flop132 is connected to one input of OR gate 134 with the other input of theOR gate receiving θ_(B) and the output of the gate being connected tothe reset input of flip-flop 66. Finally, the Q output of flip-flop 66is connected to the input of AND gate 74.

Referring to FIG. 6 as well as the timing diagram of FIG. 7, thetransfer pulse θ_(A) jam transfers the value in the scaler 76 from theprevious scan into the latch 82, as previously described. Next theclearing pulse θ_(B) clears the scaler 76 and directly resets flip-flops130 and 132 and resets flip-flop 66 through OR gate 134. The Q outputsof the flip-flops are all thus low and the Q outputs are high. With theQ output of flip-flop 130 high, the AND gate 124 is thus provided withone high input. The low Q output of flip-flop 130 is applied to theinput of AND gate 128. Finally, with the Q output of flip-flop 66 low,the AND gate 74 will be disabled. The video output of sensing unit 60 aswell as the strobe signal θs is then provided to the comparator circuit106. The image of the reference bar 102 will be relatively darker thanthe image of the solid crystal.

Accordingly, the threshold settings of comparators 108, 110 and 112 areset so that the threshold levels of all three are below the amplitude ofthe portion of the video signal representative of the reference bar 102and above the amplitude of the portion of the video signalrepresentative of the solid crystalline body 16.

Thus, as shown in the timing diagram, when the amplitude of the portionof the video signal representative of the reference bar is above thethreshold levels of the comparators 108, 110 and 112, the outputs of thecomparators 108 and 110 will be zero or negative and the output ofcomparator 112 will be positive and thus strobed through gate 120 to theinput of AND gate 128. However, since the other input to AND gate 128from the Q output of flip-flop 130 is low, the gate 128 remains disabledand flip-flop 132 remains clear.

As the amount of reflected radiation received increases at t1,indicating the edge 104 of the reference bar 102, the amplitude of thevideo signal begins to decrease. During this transition at t2 thevoltage level of the video signal first falls below the threshold levelof the comparator 108. This causes the output of the comparator 108 togo positive. This is strobed through gate 116 to an input of AND gate124. Since the other input of gate 124 from the Q output of flip-flop130 is high, the gate 124 is enabled and the positive-going transitionof the output of the gate 124 sets flip-flop 66. The Q output offlip-flop 66 thus goes high enabling AND gate 74 so that scaler 76 canbegin counting the strobe pulses θs. In this way the scaler beginscounting from the edge 104 of the bar 102. Simultaneously, the highoutput from flip-flop 66 is provided to an input of AND gate 126.

At t3, the amplitude of the video signal falls below the threshold levelof comparator 112, causing the output of the comparator to go low (azero or negative voltage) whereby both inputs to AND gate 128 are nowlow.

Finally, at t4 the amplitude of the video signal falls below thethreshold level of the comparator 110 so that the output of the lattergoes positive. AND gate 126 is thus enabled setting flip-flop 130. The Qoutput of flip-flop 130 goes from low to high, providing a high input toone of the inputs of gate 128. The latter remains disabled since theoutput of comparator 112 is not low.

The circuit 106 remains in this condition until the portion of the videosignal, representing the image on the meniscus, is provided by thesensing unit 60. At this time the amplitude of the video signalincreases due to the reduced reflected radiation received by theparticular detectors. At t5 the threshold level of comparator 110 isexceeded so that the output of the latter goes low. However, the Qoutput of flip-flop 130 remains high since a change at its set inputwill have no effect.

At t6 the threshold level of amplifier 112 is exceeded so that itsoutput goes positive. This enables gate 128 so that the flip-flop 132 isset. By setting the flip-flop 132, a positive going pulse is providedthrough OR gate 134 to the reset input of flip-flop 66. This causes theQ output of flip-flop 66 to go low and the AND gate 74 to becomedisabled. The scaler 74 discontinues counting.

Finally, at t7 the threshold level of comparator 108 is exceeded and theoutput will go to zero or some negative value. This will disable ANDgate 124 having no affect on flip-flop 66.

At the end of the scan, the transfer pulse θ_(A) is provided, jamtransfering to latch 82 the pulse count stored in scaler 76. Theremaining aspects of the operation are identical to those previouslydescribed with respect to FIGS. 3 and 4.

It will be appreciated that the count in the scaler represents thedistance between the edge 104 of the reference bar 102 to thesolid/liquid/vapor junction 38 as observed by the detectors 54. When anyshift in the junction 38 caused, for example, by convection currentjitter, a similar shift in the image of the reference bar will occur andno correction will be provided to the heating elements 18. As such thecontrol of the growth of the crystal is independent of convectioncurrent jitter.

Although the invention has been described in connection with controllingthe temperature of the melt materials, the crystal growth can also becontrolled by adjusting the pulling speed of the crystalline body bymechanism 32. In such a case the thermocouple input to the summingjunction of the controller 90 can be eliminated and the output ofcontroller 96 applied to mechanism 32.

Similarly, thermocouple 92 of the FIG. 6 embodiment can be omitted and amanually adjustable voltage source substituted for set point control 94where the level of voltage of the source required to establish steadygrowth is empirically determined by prior observation of the meniscusbehavior. In such a situation the switch 88 is set to ground, themanually adjustable voltage source 94 is adjusted to provide the desiredsteady state growth, the amplifier 86 is manually offset to ground leveland the switch 88 is then thrown to connect the output of amplifier 86to the input of the controller 90.

Although the embodiment has been described wherein (1) the referenceobject 102 is observed above the junction 38 with a portion of body 16therebetween and (2) the scanning occurs in a downward direction, itwill be appreciated that (a) the object 102 can be observed below thejunction 38 with a portion of meniscus 30 therebetween and (b) thescanning can occur in an upward direction.

It will be appreciated that a single control device can be provided tocontrol all of the heaters. Alternatively, a separate control unit canbe provided for controlling each heater or a group of heaters in orderto provide a more uniform distribution of heat to the melt material 14.Further, although the preferred embodiment of the invention has beendescribed with reference to the use of detectors 54 which sense theradiation received by each at the same time and the unit 60 whichincludes an analog shift register for storing the signals so that theycan be sequentially scanned, alternatively, each of the detectors can besequentially energized to provide a series of analog signal pulses eachrepresentative of the output of the respective detector so as toeliminate the need for an analog shift register. As a furthermodification a single detector could be used together with a scanningmirror. By moving the mirror through preselected positions so as toreflect radiation from the preselected points on at least the surfacesof the crystal body and meniscus adjacent the junction to the detectoras well as the reference object and synchronizing the movement of themirror with the output of the detector, a similar sequence of analogpulses can be provided. Alternatively, a single detector can bepositioned to straddle the image of the interface and the referenceobject to provide an analog signal, the value of which is dependent onthe position of the solid/liquid interface relative to the referenceobject. As the relative position of the interface changes, acorresponding change occurs in the output of the detector.

The invention thus described has several advantages. The growth of acrystalline body, notably a silicon body, can be monitored andcontrolled simply and easily, so that the cross-sectional dimensions ofthe growing body are within prescribed limits regardless of whetherconvection currents are present in the furnace or not. By observing thegrowth of the body electronically, and utilizing automatic controls tocontrol the speed of pulling of the crystal or the temperature of themelt material, the control is not dependent upon human observation andthus is not subject to human errors which can arise from thepsychologically stressful nature of optically observing the meniscuswith the human eye as taught by LaBelle in U.S. Pat. No. 3,870,477. Byobserving the crystalline body and meniscus along the perpendicular tothe meniscus surface where it intersects at the solid/liquid/vaporjunction when the crystal is being grown under steady state conditions,maximum contrast can be achieved between the body and the meniscus.Further, this portion of the meniscus is substantially more stable andreliable than the center portion of the meniscus, thus providingadvantages over the monitoring techniques such as described by Dohmen etal U.S. Pat. No. 3,291,650. The invention also may be used to growcrystals from a melt by techniques other than that described by U.S.Pat. Nos. 3,591,348 and 3,471,266.

What is claimed is:
 1. A method of growing a crystalline body ofselected material from a growth pool of melt so that said body has aselected cross-sectional shape for some preselected distance along itslength, said growth pool being characterized by a meniscus which joinssaid body at said growth pool said method comprising:positioningreference object means adjacent the junction where said meniscus meetssaid body; observing said body, said reference object means and saidmeniscus along an optical axis which is substantially perpendicular tothe surface of said meniscus where said meniscus meets said body and inwhich the reference object means appears spaced from the junction with aportion of said meniscus or said body spaced therebetween; determining afirst spatial position where a sharp contrast occurs between said body,and said meniscus and a second spatial position where a sharp contrastoccurs between said portion of meniscus or body and said referenceobject means; referencing said first spatial position relative to saidsecond spatial position; and controlling the speed at which said body ispulled from said growth pool or the temperature of said growth pool sothat said first spatial position can be controlled relative to saidsecond spatial position.
 2. A method in accordance with claim 1, whereinsaid material is silicon and said optical axis is approximately 11° withrespect to the horizontal.
 3. A method in accordance with claim 1,wherein said material is germanium and said optical axis isapproximately 8° with respect to the horizontal.